1. Field of the Invention
This invention relates to a small portable apparatus, and more particularly to an apparatus which eliminates noise generated when an EL device is used for illumination.
2. Description of the Related Art
In an apparatus which uses an EL device (Electro Luminescent Device), when the EL device emits light, sound (noise) is generated by structural vibrations of the EL device. This is widely known in the art. EL devices which eliminate such noise generation are disclosed in Japanese Patent Laid-Open No. 234676/1993, Japanese Patent Laid-Open No. 308910/1994 and so forth. The apparatus disclosed in the documents are characterized in that an EL device is used as a display device and a plurality of such EL devices are provided in layers. Signal voltages opposite in phase to each other are applied simultaneously to the EL devices to drive the EL devices to display and reduce vibration noise generated from the EL devices.
The EL display apparatus of Japanese Patent Laid-Open No. 234676/1993 includes an EL device wherein a pair of electrodes are formed on the opposite surfaces of an EL light emitting layer, and an AC voltage is applied between the electrodes to cause the EL light emitting layer to emit light. In the EL display apparatus, a plurality of EL devices are disposed such that the EL light emitting layers extend in parallel to each other, and between the electrodes of one of the EL devices, an AC voltage having a phase different from the phase of another AC voltage applied to at least one of the other EL devices is applied individually.
In the EL display apparatus having the construction just described, the plurality of EL elements emit light when AC voltages are applied between the electrodes provided on the opposite surfaces of the EL light emitting layers thereof. When the AC voltages are applied, the EL light emitting layers emit light and simultaneously vibrate themselves. However, the AC voltage applied between the electrodes of one of the EL devices is different in phase from the AC voltage applied to at least one of the other EL devices. Therefore, also the phases of the vibrations generated by the two EL devices are different from each other. Further, since the EL light emitting layers of the two EL devices are disposed in parallel to each other, vibrations generated by the two EL devices interfere with each other. As a result, vibrations of the entire EL display apparatus are reduced, and noise can be prevented well.
Particularly where the phases of the AC voltages applied to the EL devices are divided into two kinds, if two AC voltages having phases opposite to each other are used as the AC voltages to be applied to the EL devices, then noise can be prevented better because the vibrations originating from the two AC voltages always cancel each other.
The EL device of Japanese Patent Laid-Open No. 308910/94 includes two EL light emitting layers each having a pair of electrodes provided on the opposite surfaces thereof. The EL device is characterized in that AC voltages opposite in phase to each other are applied between the paired electrodes of the EL devices formed in two layers to cause the EL light emitting layers to emit light. Noise components by vibrations originating from the two AC voltages cancel each other because the two AC voltages are opposite in phase to each other, and noise is prevented much better.
However, the EL devices disclosed in Japanese Patent Laid-Open No. 234676/1993 and Japanese Patent Laid-Open No. 308910/1994 which eliminate noise generation have a problem in that, since EL devices each of which includes a pair of electrodes and an EL light emitting layer are layered, not only the thickness increases, but also the power consumption increases.
Further, an EL device is often used for illumination of a liquid crystal display unit for a personal computer or an audio apparatus. However, since the level of noise is low, the noise does not bother the user of the EL device in a situation of use in which various sounds are generated therearound such as at home or in an office. However, where an EL device is used with a small portable apparatus such as a portable telephone set and emits light at a place very close to an ear of the user such as at the distance of 1 cm or 2 cm, noise from the EL device comes over to the ear.
Such noise generated from an EL device cannot be avoided from the structure of the EL device, and from restriction to the dimensions of the outer profile and the power consumption, a vibration preventing sheet for absorbing vibrations is usually adhered to the rear surface of the EL device or a like countermeasure is taken to reduce the noise.
Where a vibration preventing sheet is used to reduce noise of an EL device, if the thickness of the noise preventing sheet is small, then the noise reduction effect is low, but if the thickness of the noise preventing sheet is increased, then the size and the weight of the apparatus are increased although the apparatus is a small portable apparatus. The increase in volume and weight has a very great influence with a portable apparatus of several tens grams. Further, although the noise preventing sheet can reduce noise, it cannot eliminate the noise, and besides, an increase in cost is invited by adhesion of the vibration preventing sheet.